This invention relates to photoflash lamps and particularly to photoflash lamps containing a filamentary combustible material which is ignited to produce actinic light.
A typical photoflash lamp comprises an hermetically sealed glass envelope containing a quantity of filamentary combustible material, such as fine strands of zirconium or hafnium, and a combustion-supporting gas, such as oxygen, at a pressure well above one atmosphere. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of an electrical current through the lead-in wires which incandesces the filament to ignite the ignition paste which in turn ignites the combustible metal in the envelope. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on an anvil wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible metal disposed in the lamp envelope.
The combustible fill material comprises a major constituent of a photoflash lamp, and consequently the cost of the fill material has a significant impact on the cost of the finished lamp. In the early history of photoflash, the combustible material employed in the lamps generally comprised an extremely thin metallic foil and/or a fine metallic wire, both of which proved relatively expensive. In the case of the foil, which was typically aluminum having a thickness in the order of 0.000015 to 0.000020 inch, in order to obtain such extremely thin foil, the aluminum was rolled into thin sheets and then beaten between heating forms, in the manner known for the preparation of gold foils, until the desired thinness was obtained. Such a slow and costly process made the aluminum foil quite expensive. Likewise, the wire used in flash lamps (typically aluminum or aluminum-magnesium alloy) underwent an involved and expensive wire-drawing process in order to be drawn down to the required size, typically 1.5 mils or less in diameter, for flashlamp use.
Accordingly, with the object of reducing the cost of flashlamps, further development work in this field resulted in the production and use of shredded foil as a substantially less expensive filamentary combustible material for use in flashlamps. Typically, the shredded foil is formed by feeding a thin sheet of the combustible metal material into a suitable cutting machine, such as the reel type shredder shown in U.S. Pat. No. 2,699,831 for example. The thin strands of foil produced by the cutting machine are then introduced and distributed into the lamp envelopes with the aid of a moving current of air, such as described, for example, in U.S. Pat. Nos. 2,772,703 and 2,862,529. Aluminum foil has been used for this purpose, although more recently, zirconium and hafnium have been found to provide significant photometric advantages. Typically, zirconium or hafnium foil having a thickness of about 0.95 mil is shredded to provide four inch long strands having a width of about 1.2 inch. Thus, the cross-section of each strand is rectangular but distorted somewhat along one or two edges by the shearing action of the cut.
Although providing subminiature flashlamps with high light output characteristics, shredded zirconium foil and, more particularly, hafnium foil, are quite expensive and constitute an appreciable portion of the cost of the lamp. While zirconium and hafnium metals in themselves are relatively expensive, a greater part of the cost arises from the large amount of work in reducing the materials to a foil less than one mil thick. For example, the price of zirconium foil may be about twice the basic price of the zirconium metal, and the cost of hafnium foil may exceed four times the basic metal price.
In addition to these cost considerations, a variety of strand configurations have been employed to improve the distribution and ignition characteristics of the shredded combustible foil in smaller size lamps. For example, in order to improve useful light output by minimizing molten droplet size and the light-quenching wall contact effect of the long four inch strands in subminiature lamps, a copending U.S. Patent Application Ser. No. 179,056, filed Sept. 9, 1971, now U.S. Pat. No. 3,895,902, and assigned to the present assignee, describes the use of short shreds (e.g., strand lengths of 0.200 inch) arranged in a manner to keep the bulk of the combustible material away from the wall of the lamp envelope. Accordingly, the use of the shorter strand lengths significantly increases the efficiency of combustion in lamps of less than one cubic centimeter internal volume by substantially reducing the lateral contact areas between the combustible material and lamp wall. In addition the smaller shreds produce smaller molten droplets during the combustion process. A manufacturing drawback to the use of short shreds, however, is that the production of such shreds by the use of slot foil on the shredding machine can prove to be a difficult mechanical set up for obtaining uniform results. On the other hand, in the case of flashlamps having an internal envelope volume substantially larger than 1 cc., the short shred configuration would appear to be somewhat impractical since support of the desired shred distribution by the envelope wall would be virtually impossible with standard loadings.
Another strand configuration directed toward optimizing combustion in subminiature lamps by supporting the shredded foil away from the envelope wall is described in U.S. Pat. No. 3,630,650. According to this arrangement, shreds of 100 mm. (about 4 inches) in length are crumpled to have a random configuration of from 8 to 50 sharp bends per strand to effect point contact between the filling and the envelope wall. When distributed in the lamp, each 100 mm. shred forms a ball with a cross-sectional area of from 40 to 8 mm.sup.2. This crumpled foil packing geometry provides significantly improved light output over that obtainable using the old intermingled loop configuration, however, it does not permit the degree and uniformity of droplet size control attained by strand configurations such as that described in U.S. Pat. No. 3,792,957 of Bouchard et al.
The above-mentioned Bouchard et al patent describes a flashlamp having a filamentary combustible in which each of the strands comprises substantially uniform periodic variations adapted when burning to eject a predetermined number of molten droplets per unit length of strand. According to one embodiment, each strand has a coiled configuration with a diameter of from 0.010 to 0.030 inch and a pitch of from 20 to 300 turns per inch. In an alternative embodiment, each strand is crimped to provide a plurality of substantially straight segments of approximately equal length interconnected at sharp bends in the strand. The length of each segment may be from 0.010 to 0.060 inch and the bend angle may be from 60.degree. to 165.degree.. When ignited and burning in a flashlamp, such crimped or coiled strands produce a greater number of molten droplets without altering the quantity of combustible material, fulminating material, or gas fill. In particular, the uniformity of the periodic variations in the strand configuration causes droplets of substantially uniform size to be ejected by centrifugal force when the strand is burning and the dimensional constraints on the configuration provide for a significant reduction in size (and thus increase in number) of the ejected droplets. Operation of flashlamps employing such combustible foil configurations has been observed to provide significantly improved light output characteristics. This approach, however, also has the disadvantage of being relatively difficult and costly to implement on high-speed production machinery.